In cellular radiotelephone systems, handoff of a mobile is required to maintain communication to the mobile as it moves from cell to cell in the system. Handoff can be defined as the process of transferring a call in progress from one RF coverage area to another in coordination with the movement of the mobile. It is also the process whereby a call is transferred to another channel within a RF coverage area because of interference within the coverage area. The process of handing off a call in progress is one of the most delicately balanced functions related to cellular radiotelephone systems because it requires a high level of coordination among the various system processing elements to ensure successful operation. Failure to hand a call off at the proper time generally results in a reduction in the call quantity, interference with neighboring coverage areas and even the undesired termination of the call.
Current analog cellular radiotelephone systems require that the system continuously monitor the quality of every call which is operational on the system. The system must recognize when the quality of a call falls below a predetermined threshold in a particular coverage area and must also determine what other coverage area can satisfactorily handle the call. Once a more suitable coverage area is identified, the system sends instructions to the mobile directing it to another channel. The mobile confirms that it is leaving its current channel, tunes to the new channel, synchronizes to the new channel and begins transmitting thereby confirming that it has arrived on the new channel.
In digital cellular radiotelephone systems, the procedure is modified somewhat in that the mobile is capable of measuring other channels as instructed by the system as well as its current channel and also that the mobile reports this information back to the system. These measurements consist of signal strength only and are relative as the measured results are likely to vary considerably due to varying environmental conditions. Because of this and because the mobile cannot determine if the measurement is that of an interferer or the correct channel, the system must scan for the mobile in the cell selected by the mobile.
Although in digital cellular communications systems the mobile is capable of measuring other channels and reporting back to the system, the scan the target cells perform still occurs after the quality of the call has fallen below a predetermined handoff level in a particular coverage area. Thus, by using the mobile to determine target cell candidates, the number of candidate target cells is decreased but no improvement on call quality during handoff is made.
Thus, a need exists for a handoff procedure which will initialize the handoff procedure before the predetermined handoff threshold.